Voltage references which provide relatively constant reference voltages have many applications in integrated circuits. As one example, a voltage reference may be used to bias a bipolar transistor to a given level of conduction. As another example, a voltage reference may act as a clamp to prevent the voltage across a device from exceeding a specified voltage.
FIG. 1a illustrates a conventional voltage reference having a pair of points or terminals A and B across which is a reference voltage V.sub.AB. This voltage reference consists of a Schottky diode D and a diode-connected NPN transistor Q. In particular, terminal A is connected to the anode of diode D. Its cathode is connected to the interconnected base and collector of transistor Q whose emitter is connected to terminal B so that all of the current I flowing between terminals A and B flows through diode D. Reference voltage V.sub.AB is the sum of the voltages across diode D and the base-emitter junction of transistor Q.
The value of voltage V.sub.AB is affected by the intrinsic semiconductor resistances associated with elements D and Q when the voltage reference is part of an integrated circuit. These internal resistances are explicitly shown in FIG. 1b. The cathode of diode D has an intrinsic resistance r.sub.D. The anode of diode D is an electrical conductor, typically aluminum, and therefore has no intrinsic semiconductor resistance. The base, collector, and emitter of transistor Q have respective intrinsic resistances r.sub.B, r.sub.C, and r.sub.E.
Upon taking the internal resistances into account, voltage V.sub.AB for this reference can be expressed as: EQU V.sub.AB =(kT/q)1n(I/I.sub.SS)+Ir.sub.D +Ir.sub.B /(.beta.+1)+.phi..sub.BE +Ir.sub.E ( 1)
In the first term which represents the voltage drop across the rectifying junction in diode D, k is Boltzmann's constant, T is the temperature of the voltage reference, q is the electronic charge, and I.sub.SS is the saturation current of diode D. .phi..sub.BE is the voltage drop across the base-emitter junction of transistor Q. In the remaining terms which are largely self-explanatory, .beta. is the amplification of transistor Q--i.e., the ratio of collector current to base current. I.sub.S, .beta., and the intrinsic resistances increase with temperature, while .phi..sub.BE decreases with temperature. The rates of change of these parameters with temperature normally cause V.sub.AB to decrease slightly with increasing temperature.
The variation of voltage V.sub.AB with temperature is often a desirable feature since it acts to compensate for temperature-induced changes in characteristics of the portion of the integrated circuit using the voltage reference. However, the V.sub.AB temperature variation is not great enough to meet temperature compensation needs in certain applications. Operation is detrimentally affected. Moreover, the impedance of the reference of FIGS. 1a and 1b is unduly high in some applications.
Voltage references of the foregoing type are used in numerous kinds of bipolar digital circuitry. Transistor-transistor logic (TTL) is one example. Turning to FIG. 2a, it shows a conventional arrangement for the common NAND logic structure which is the basis for TTL circuitry. In this TTL gate, a set of logical input voltage signals represented here by input voltages V.sub.I1, V.sub.I2, and V.sub.I3 are provided to the corresponding emitters of a multiple-emitter NPN input transistor Q1. Its base is connected to a current source consisting of a resistor R1 connected to a source of a high supply voltage V.sub.CC.
The Q1 collector is connected to the base of an NPN phase-splitting transistor Q2. Its collector is connected to a current source formed by a resistor R2 tied to the V.sub.CC supply. The Q2 collector is further connected to the base of an NPN transistor Q3 whose emitter drives an NPN output pull-up transistor Q4 and is also coupled through a resistor R3 to the Q4 emitter. The interconnected collectors of the Darlington pair Q3 and Q4 are connected to a current source consisting of a resistor R4 tied to the V.sub.CC supply.
The Q2 emitter is connected by way of a line 10 to the base of an NPN output pull-down transistor Q5 whose emitter is connected to a source of a low supply voltage V.sub.EE. A logical output voltage signal V.sub.0 is provided from the interconnection of the Q5 collector and the Q4 emitter. A passive output pull-down circuit 12 consisting here of a resistor R5 is connected between the V.sub.EE supply and the Q5 base by way of line 10.
The connection of the Q1 collector to the Q2 base was originally the distinguishing feature of TTL circuitry. More recently, the definition of TTL has loosened somewhat. TTL today generally means a family of bipolar devices having certain input/output characteristics. The internal circuitry of current TTL gates may contain some circuitry that would strictly fall in another logic family such as diode transistor logic, integrated injection logic, and the like. Even the Q1 collector-Q2 base connection may be absent as long as the necessary input/output conditions are satisfied within the general realm of TTL design.
FIG. 2b shows the input portion of a conventional TTL-type NAND gate configured according to the more modern definition of TTL. In FIG. 2b, the combination of PN-junction diodes DE1, DE2, and DE3 and the base-emitter junction of an NPN input transistor QC1 replaces transistor Q1. Resistor R1 is connected between the V.sub.CC supply and the junction of the QC1 base with the anodes of diodes DE1-DE3 whose cathodes respectively receive inputs V.sub.I1 -V.sub.I3. The QC1 collector is here connected to the emitter of an NPN transistor Q6 whose collector is tied to the V.sub.CC supply. The Q6 base is connected to the Q2 collector so as to form (in conjunction with transistor QC1) a "kicker" circuit that speeds up the switching of phase splitter Q2.